118 Transactions. 



Art. VIII. — Geology of the Oamaru- Papakaio District. 

 By G. H. Uttley, M.A., M.Sc, F.G.S., Principal. Scots College, Wellington. 



[Bead before the Wellington Philosophical Society, 19th September, 1917 ; received by 

 Editors, 31st December, 1917 ; issued separately, 24th May. 1918.] 



Contents. 



1. Description of the Area. 



II. Historical Summary. 



III. Aim of this Paper. 



IV. Description of the Sections. 



1. Devil's Bridge. 



2. Grant's Stream. 



3. Water-race Creek. 



4. Lanclon Creek. 



5. Flume ( 'reek. 

 V. Summary. 



I. Description^op^thk Area. 



The area (about twenty square miles in extent) with which this paper is 

 mainly concerned is the north-eastern corner of the Province of Otago. It 

 is bounded on the north-east by the val'.ey-plain of the Waitaki Biver, on 

 the south-east by the sea, on the south by a line drawn east and west 

 through one mile north of Oamaru, and on the west by a line drawn due 

 south through Peebles (:-ee fig. 1). 



A considerable portion of the district, particularly between the township 

 of Papakaio and Oamaru, is capped by heavy river-gravels and silts ; these 

 are evidently the remnant of a formerly extensive plain which sloped to the 

 south-south-east, as the general trend of the streams that drain the area 

 is in that direction. This former surface has been sculptured into well- 

 rounded ridges and hills, attaining an elevation of 650 ft. near Papakaio in 

 the north, and falling to the 350 ft. level in the south-east. In the north- 

 western part of the district the Ngaparan coal-grits, mudstones, and green - 

 sands crop out, and a ridge of quartz gravels in this locality attains a height 

 of over 1,000 ft. Noticeable features of the country south of this ridge are 

 the two well-marked depressions in the vicinity of Tabletop Hill and on the 

 Ardgowan Estate. The Oamaru Creek in flowing over the softer greensands 

 and tuffs of the former locality has by rapid lateral erosion widened its basin 

 considerably, leaving, however, two limestone-capped flat-topped hills stand- 

 ing prominently above the surrounding low-lying area. The stream on 

 leaving this open tract of country becomes much constricted as it flows over 

 the limestone, and at the Devil's Bridge it passes beneath a natural bridge 

 of limestone before it reaches the Ardgowan area, where the softer Hutchin- 

 sonian and Awamoan rocks have undergone extensive denudation, forming 

 the second depressed area. The stream then enters the volcanic area north 

 of Oamaru, and its narrow bed is flanked by precipitous cliffs of basaltic 

 rock. 



